1. Field of Invention
The invention relates to a multilayer material for sliding elements having a backing and an overlay of lead bronze. The invention also relates to a process for producing sliding elements in which an overlay of lead-tin-copper is applied to the prefabricated semi-finished product by electroplating, as well as to means for carrying out the process.
2. Description of Prior Developments
Sliding elements should be understood to mean all types of bearing liners as well, inter alia, as pistons and piston rings. Overlays of multilayer materials for sliding elements are generally produced by electroplating in appropriate electrolyte baths. There have hitherto generally been used for this purpose fluoroborate-containing baths, which exhibit a number of disadvantages, however.
When baths are used, organic constituents and tin oxidation inevitably cause impurities to arise, which detract from the quality of the electroplated layer. As a rule, owing to their greater weight these impurities settle on the bottom of the tank and do not affect electroplating unless the bath is moved. However, if circulation of the bath cannot be effected for these reasons, a concentration gradient of the metal ions provided for plating arises. Since the sliding elements to be plated, such as bearing liners for example, are stacked one above the other in receptacles, an increase in layer thickness from top to bottom will inevitably occur.
Such concentration gradients could be prevented by constant movement of the bath, though a prerequisite of this would be that no impurities are whirled up or that the bath is filterable. The tetravalent tin oxide, which is present in fluoroborate baths, is generally not filterable, however. Moreover, undesired tin oxidation would be encouraged by the increased oxygen supply from the air.
A further disadvantage of known baths consists in the fact that, despite the provision of a thief cathode in the electroplating bath, it is impossible to prevent currentless deposition of bearing metals on the bearing backing during the electroplating process. Owing to the high copper concentrations, for example in electrolytes for ternary layers, copper is deposited currentlessly and without check on the bearing backings of the bearing liners, which process is known as cementation. This copper layer exhibits only limited bonding to the steel backing and thus has a tendency, after tinning, towards bubble formation and flaking. On the one hand, this entails a material loss and on the other hand, the operability of the bearing may be impaired by flaking of the cementation layer.
The current densities used are limited to 2.2 A/dm.sup.2 in known electroplating baths. Although higher current densities shorten the electroplating process, the deposition rates of the metals diverge within the bearing liner surface and receiving stack, such that the deposited layer no longer has the desired composition. Stable deposition is not possible, therefore, at higher current densities.
Some overlays, especially ternary layers, produced with known baths exhibit considerable variations in thickness, which may possibly necessitate post-machining. Furthermore, the tin is not evenly distributed in the overlay, which may cause aggregation and coarse crystalline deposits, known as tin-agglomerations. This inhomogeneous structure of the overlay promotes tin diffusion, which occurs when the sliding element is exposed to relatively high heat levels during operation, such that such overlays may only be applied to an intermediate layer, such as a nickel barrier for example, which prevents tin diffusion into the lead-bronze layer thereunder, as described, for example, in E. Romer "Werkstoff und Schichtaufbau bei Gleitlagern" ("Material and Layer Structure in Plain Bearings"), an offprint from ZFW Zeitschrift fur Werkstofftechnik ("Journal of Materials Technology"), volume 4, issue 7, Verlag Chemie Weinheim/Bergstra.beta.e 1973. Only through this additional measure has it hitherto been possible to improve corrosion resistance and to prevent detachment phenomena in the overlay, which may lead to bearing damage. Moreover, the hardness of these known overlays and thus their wear resistance are inadequate.
The inhomogeneous structure of the overlay connected with conventional electroplating baths was deliberately exploited for diffusion in the process according to DE 41 03 117 C2. A layer of a binary or ternary basis alloy with a relatively low tin content was-applied by electroplating to a backing layer, as desired. A further layer with a markedly higher tin content was then applied to this layer. By time-controlled thermodiffusion treatment, the tin was allowed to diffuse into the base alloy, until it was distributed in the desired amount at the future running surface. Since the inhomogeneous, coarse crystalline structure of the overlay is still present even after heat-treatment, the diffusion processes may continue in the operational stage of the sliding element. To prevent negative effects at the interface with the backing layer, a diffusion barrier layer is therefore provided.
From DE-OS 27 22 144 it is known to use an alloy, of which more than 6 to 10 wt. % is copper, 10 to 20 wt. % is tin and the rest is lead, as a soft metal bearing alloy for multilayer plain bearings. This alloy may, inter alia, be applied by electrodeposition, an intermediate layer of nickel being provided as the diffusion barrier. This known alloy, which is provided with conventional electrolyte baths, exhibits coarse tin distribution.
"Galvanisches Abscheiden von Zinn-Blei aus Fluoroborat und fluoroboratfreien Elektrolyten" ("Electrodeposition of tin-lead from fluoroborate and fluoroborate-free electrolytes") by H. van der Heijden in "Metalloberflache" ("Metal surface") 39(1985) 9, pages 317 to 320, describes the plating of electronic components. It has been shown that tin and lead are easily soluble in various organic sulphonic acids of the general formula RSO.sub.1 H, wherein R represents an alkyl group. The sulphonic acids are described as completely stable during electrolysis. As little mention is made of the use of such fluoroborate-free baths for the electroplating of sliding elements as of additives suitable for and necessary in the production of sliding elements.
Aqueous, acidic solutions for the electrolytic deposition of tin and/or lead/tin-alloys are known from DE 3902 042 A1. These binary electroplating baths are used as corrosion protection, e.g. for printed circuit boards and batteries, a relatively soft layer being deposited which is not wear-resistant and cannot therefore be used for sliding elements. In addition to metal salts, inhibitors and free alkane sulphonic acid, these known electroplating baths comprise brighteners, which render impossible the use of these baths as ternary baths, for example.
It has been shown that, when, for example, copper salts are added, only a maximum of 1% copper can be deposited, because the bath composition is essentially conformed to tin.
JP 02-93 096 (Pat. Abstr. of Japan, C-732, Vol. 14/No. 294) describes a process for producing sliding elements, in which the overlay of Pb--Sn--Cu is applied to the prefabricated semi-finished product by electroplating. For this purpose, a fluoroborate-free electrolyte is used without brightener but with free alkyl sulphonic acid and non-ionic wetting agents. With these known electroplating baths, however, again only coarse deposition of the tin is achieved, the layer structure generally being irregular. The considerable demands made of plain bearings are not wholly satisfactorily fulfilled.
The additives necessary for the production of sliding elements are not mentioned, such that in the end it is once again only possible to use current densities of up to 3 A/dm.sup.2.